In many aerospace applications, particularly airplane manufacture, whenever a rivet or other fastener will be installed in an assembly, a sealant must be applied around the fastener at the same time. Currently, aircraft sealants are two-part sealants that need to be mixed accurately and used within a certain amount of time. The mixed sealant is hand applied to fasteners and the “wet” fasteners are then installed. Excess sealant is wiped off and disposed of along with unused sealant.
The labor spent applying sealant to rivets and fasteners in aircraft manufacture and repair is a significant cost and time factor. To put the wasted time and effort into perspective, the C-17, the largest military aircraft currently in production, has over 1.4 million rivets and fasteners in each aircraft. It is estimated that there are on the order of tens of millions of wet installed fasteners going into military and commercial aircraft per year. Eliminating or greatly reducing the labor spent in wet fastener installation could save tens of thousands of labor hours and millions of dollars on the manufacture of just one C-17 aircraft, for example. The case is the same with commercial aircraft platforms such as, for example, the Boeing 747 or Airbus A380.
In addition, the process variability associated with “wet” installations is large. The amount of sealant applied, and areas of the rivet/fastener coated or uncoated vary widely with the installer. Excess sealant is often applied, resulting in additional time and expense for cleanup of the excess sealant. Further, prior art wet installation creates hazardous waste disposal problems because excess and unused sealant often contains toxic materials such as Chromium.
A number of attempts have been made at developing automated or semi-automated methods for coating rivets and fasteners over the past 30 years, but all were either unsuccessful or suffer from significant shortcomings.
An improved fastener for use in airplane manufacture was described by Bray et al., in “Self-Sealing Fastener,” U.S. patent application Ser. No. 12/034,636 (filed Feb. 20, 2008) (hereinafter “Bray '636”), which is incorporated herein by reference. While the fastener design described in Bray '636 does represent a significant improvement over the prior art, there are a number of areas in which additional improvements are desirable, especially in areas that are significant from the standpoint of commercial viability.